Jack's the heart of the band and Brady is the soul. Jack is impulsive and is willing to do anything to ensure the happiness of the people close to him. He has the wanderer's spirit, and even though he really wants to get to Boise because he thinks that their next big thing is there, he also knows that the right thing for the band might be to make Brady a little bit happier and to bring Silas back into the fold.
Brady is cool and contemplative, and he always tries to see the bigger picture. He also wants the people around him to be happy but he doesn't need to be directly involved in their happiness. As deeply as he feels for Silas he wants what is best for him first and foremost, and as far as Brady can see what Silas needs is time to figure things out. I've seen it said a few times that people think Brady is being brave or that he has more faith in the "if you love it let it go" then one would usually have, but I find his motivation to be slightly different. He just wants Silas to find happiness, wherever it may be.
Or at least, that's what he keeps telling himself.
Stacey and Theo are pretty awesome too, but you'll have to stick around if you want to see why.